Assembled batteries (also called batteries or battery packs), comprising lithium ion batteries, nickel hydrogen batteries, or other secondary batteries as unit battery cells, with a plurality of such unit battery cells connected in series, are becoming increasingly important as high-output power supplies for hybrid automobiles and mounted power supplies of other vehicles, and as power supplies for personal computers and portable communication devices. In particular, assembled batteries comprising as unit battery cells a plurality of series-connected lithium ion batteries, which are lightweight and can have high energy densities, are anticipated as favorable for use as high-output mounted power supplies for vehicles.
In a system which controls charging and discharging of this type of assembled battery, mounted on a vehicle as a power supply for motor driving, various methods are employed so that overcharging of the assembled battery does not occur during charging processing performed on the assembled battery in a predetermined time period (for example, during brake operation).
For example, in Patent Reference 1, an assembled battery control apparatus is disclosed with a configuration in which Zener diodes are connected in parallel as bypass circuits to each of the plurality of unit battery cells constituting the assembled battery. In Patent Reference 1, it is stated that application to each unit battery cell of a voltage equal to or higher than the Zener voltage of the Zener diodes can be prevented during charging of the assembled battery by a control apparatus with this configuration. Similarly, Patent Reference 2 and Patent Reference 3 also disclose assembled batteries (storage batteries) configured with Zener diodes (constant-voltage diodes) connected in parallel to each of a plurality of series connected unit battery cells.
Further, in the above-described Patent Reference 1, an assembled battery control apparatus is disclosed with a configuration in which, as an overvoltage detection circuit in addition to the above bypass circuit, second Zener diodes, with Zener voltages different from those of the first Zener diodes constituting the bypass circuit, and light-emitting diodes are connected in parallel to each of the plurality of unit battery cells constituting the assembled battery. It is stated that by means of a control apparatus of this configuration, the charging capacity can be made substantially uniform for each of the unit battery cells by the bypass circuit, and moreover unit battery cells in an overvoltage state can be displayed by the overvoltage detection circuit.                Patent Reference 1: Japanese Patent Laid-open No. H11-332115        Patent Reference 2: Japanese Patent Laid-open No. S62-221826        Patent Reference 3: Japanese Patent Laid-open No. 2002-238179        
However, in a circuit configuration such as described in the above Patent References 1 to 3, in which one or two Zener diodes are connected in parallel to each of the battery cells constituting an assembled battery, when a state occurs during charging in which a high voltage exceeding a predetermined upper-limit voltage is applied to a unit battery cell for some reason, it cannot be said that consequent effects on the unit battery cell (and consequently the entire assembled battery) can be reliably prevented. In particular, in the case of an assembled battery in which unit battery cells are lithium ion batteries (and in particular nonaqueous electrolyte type lithium ion batteries), a fail-safe function, which reliably achieves prevention in advance of the occurrence of problems such as an overcharged state in which lithium is precipitated on the negative electrode, is sought.